The invention relates to a thermal spraying apparatus and also to a thermal spraying process for coating a substrate.
“Thermal spraying” has been established for a long time in the manufacture of single parts and in industrial series production. The most common thermal spraying processes which are in particular also used in series production for the coating of the surfaces of large numbers of substrates are, for example, flame spraying with a spray powder or with a spray wire, arc spraying, high velocity flame spraying (HVOF), detonation spraying or plasma spraying. The above-named list of thermal spraying processes is certainly not exhaustive. On the contrary, the person averagely skilled in the art is familiar with a large number of variations of the listed processes, and of further processes, for example special processes such as flame spray welding.
In this connection thermal spraying has opened up broad areas of use. One can certainly estimate that thermal spraying as a surface coating process is the coating technology with probably the largest area of use with regard to its possibilities of use. Thus a delimitation of the areas of use of the spraying processes listed above does not seem particularly sensible because the areas of use can overlap one another.
In this context the spectrum of use of the different thermal spraying processes ranges from the improvement of the performance of stressed surfaces against mechanical stresses, such as friction for example, against high temperatures, against chemical attacks on the surface to aesthetic use such as for example the improvement of the appearance of objects of personal use. The range of substrates whose surfaces are routinely coated by thermal spraying today is correspondingly broad. Typical examples are parts of all kinds which are subject to wear and tear, components of combustion engines such as the running surfaces of cylinders in petrol or diesel engines, pistons and piston rings of these engines, the application of heat insulation layers onto turbine parts of turbines for use on land or in the air, the coating of hydraulic pistons, kitchen utensils, such as pots or pans, and much more. All materials which can be melted or at least become viscous or melted at the surface by the supply of energy can be considered as spraying material in the form of spray powder or spray wires, for example. Practically all kinds of materials can be coated in this manner, for example wood, glass, ceramics, metals, steels and alloys, but also plastics and textiles.
Special applications very often demand the application of a coating which is constructed from a plurality of individual layers sprayed on top of one another. Thus, by way of example, a coating which is intended to protect a turbine blade against the extreme conditions in the turbine in the operating state can consist of a bond layer or a connecting layer, which guarantees a good connection to the substrate of a layer to be applied. An anti-diffusion layer can be applied on this which prevents a diffusion of alloy components out of the substrate or vice versa for example. A special hard layer can be provided as a further layer of the surface layer which protects against mechanical and chemical attacks in particular and finally a heat insulating layer can be applied as a covering layer, for example on the basis of zirconium oxide for protection against the high temperatures which prevail in the operating state of the turbine.
As the above-named example impressively shows, one of the great advantages of thermal spraying is that a coating can be applied from a layer system of a plurality of individual layers which can be sprayed from completely different materials and thus can also fulfill different functions. Furthermore it is also possible in special cases to combine different thermal spraying processes when applying a layer system, so that a specific layer of the layer system can be applied by means of a plasma spraying process, for example, and another layer of the same layer system, for example a final thermal insulating layer, is sprayed on by means of a HVOF process. It is even possible to combine a thermal spraying process with another coating process, for example with a thin layer process such as PVD (physical vapor deposition) or CVD (chemical vapor deposition) or with an arc vaporization process for example.
A typical example is the application of a dual-layer system with a plasma spraying process wherein the two layers have to be sprayed with two different spray powders. Thus it is known for example to apply a coating to a substrate as protection against wear which additionally has to satisfy certain aesthetic demands. The actual wear protection layer can have excellent wear protection characteristics, for example, and can also, for example, have a gleaming white color following the application, which is desired for aesthetic reasons. However, it can happen that the wear protection layer has very bad adhesive characteristics on the substrate to be coated. Therefore it is current practice, prior to the application of the for example white, aesthetically pleasing wear protection layer, to initially apply a bond layer made of another material directly to the surface of the substrate, i.e. using a different spray material than the spray powder from which the wear protection layer is formed. The spray powder for the bond layer is selected in such a way in this arrangement that, on the one hand, the spray powder has very good adhesive characteristics to the substrate and, on the other hand, so that the white wear protection layer adheres very well to the bond layer. As a result one has a coating comprised of a dual-layer system which as a whole adheres very well to the substrate and on the other hand offers a very good wear protection against mechanical attacks on the surface, with the coated surface simultaneously having an aesthetic white appearance.
A decisive disadvantage in the manufacture of these and other multiple layer systems, in particular in series production using the thermal spraying process known from the prior art and using the known thermal spraying apparatuses used for thermal spraying, is that the spraying procedure has to be interrupted during the coating process, at the transition from the spraying of one individual layer to the spraying of the next layer which has to be sprayed using a different spray material or using a different spraying process. This is because the spray pistol has to be exchanged, in order to change the type of spray pistol, and/or because another spray wire has to be installed. Depending on the specific thermal spraying apparatus or the specific thermal spraying process which is used, it can also be necessary to interrupt the spraying process to spray on a further layer, or to install the substrate in another thermal spraying apparatus, in order to then apply the further layer by means of the other thermal spraying apparatus.
The problems which were explained previously by way of example using the spraying process known from the prior art and the known thermal spraying apparatus lead to a considerable complication of the coating procedure as a whole. This requires additional equipment and results in the tying up of working resources, leading in particular to an increase in the working time during coating and thus to a clear cost increase for the corresponding products.
At least in some cases, i.e. in some quite special cases, namely in cases which relate to the thermal spraying of coatings made of a plurality of individual layers on the surface of a substrate by means of two or more different spray powders, attempts were made to avoid these problems by, for example, providing two or more different feeds in a plasma spraying apparatus which are associated with different powder supplies instead of a single feed for one spray powder.
In the above-named plasma spray apparatus, a plasma beam is produced by means of a plasma spray pistol, into which a spray powder is introduced by means of the feed, which is, for example, melted in the plasma flame of the plasma beam and is thrown onto the surface of a substrate which is to be coated, so that a surface layer made of the material of the spray powder forms on the substrate.
If now, by way of example, two feeds are provided for the spray powder, which can be fed with spray powder from two different spray powder supplies, then it is possible in this way to apply two (or more) different layers one after the other onto the surface of a substrate and thus to form a coating of a multiple layer system without changing the spraying process. A corresponding known thermal spraying process can for example be carried out in the following manner. A shutoff device is provided between the spray powder supplies, in which a certain spray powder is stored for supplying the feeds with spray powder, and the corresponding feed itself, so that the supply of the feed with spray powder can either be enabled or prevented.
To illustrate the process, reference will be made in the following to the dual-layer system already mentioned above, which comprises a bond layer which for example has a black color due to the spray powder used and a wear protection layer applied to this which should have a gleaming white color for aesthetic reasons.
For the application of this dual-layer system by means of a plasma spraying apparatus, a plasma flame is ignited initially in a spray pistol which is directed towards the substrate which is to be coated, so that spray powder which has been introduced into the plasma flame and sintered by the plasma flame is thrown onto the surface of the substrate to form a layer.
For the formation of the dual-layer system, the connection between the spray powder supply which contains the spray powder for the formation of the white wear protection layer is first interrupted so that no spray powder for the formation of the wear protection layer can be supplied to the corresponding feed. However the connection between the feed and the powder supply which contains the spray powder for formation of the bond layer is open, so that the powder for the formation of the bond layer can be supplied to the plasma flame.
By this means in a first process step, the bond layer can initially be applied to the substrate. When the application of the bond layer is complete the feeding of the spray powder to the feed from the spray powder supply is discontinued, so that no further spray powder can any longer be supplied to the corresponding feed from this powder supply.
Thereafter the connection is established between the feed which is associated with the powder supply which contains the spray powder for the formation of the white wear protection layer and the powder supply so that the spray powder for the formation of the white wear protection layer is supplied to the plasma flame and correspondingly the white wear protection layer can be applied on the previously applied black bond layer. Thus it is indeed possible using this apparatus known from the prior art to spray a dual- or multi-layer system using different spray powders, without interrupting the spraying process, i.e. without switching off the plasma flame and/or exchanging a feed for the spray powder and/or installing the substrate into another plasma spraying apparatus for the formation of a second layer.
A considerable disadvantage of this known plasma spraying apparatus is however that in a spray powder feed itself or in a connection line between a spray powder supply and the feed even after an interruption of the connection between the spray powder supply and the associated feed the remains of the corresponding spray powder still exist. The result of this is that by means of the considerable negative pressure which the plasma flame produces, these remnants of the spray powder are sucked out of the feed during further spraying together with another spray powder which is supplied to the plasma flame as described above from another feed for the spraying of a further layer, and thus the spray powder, which is actually intended for the formation of a further layer, is contaminated. This means that the further layer contains certain constituents of the spray powder which were actually intended solely for the formation of a first layer.
It is obvious that pollutants such as these can have considerable negative consequences. If for example pollutants are introduced into the white wear protection layer described above by that powder which should actually only form the black bond layer, the white covering layer will not have the lovely aesthetic white color but rather be dyed more or less grey or contain black spots. If aesthetic qualities play a certain role in a product, then a product with a surface which has been polluted in this manner is of course unusable and has to be rejected.
However pollutants in a layer can also lead to a clear deterioration of the mechanical, chemical, physical or thermal characteristics of the polluted layer. Even small amounts of pollutants can, in special cases, lead to certain layer characteristics deteriorating so dramatically that the coating as a whole no longer has the desired characteristics and the coated part is unusable and has to be rejected.